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Tropical Transpacific Shore Fishes

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Title:Tropical Transpacific Shore Fishes
Authors:Robertson, D Ross
Grove, Jack S.
McCosker, John E.
Date Issued:Oct 2004
Publisher:University of Hawai'i Press
Citation:Robertson DR, Grove JS, McCosker JE. 2004. Tropical transpacific shore fishes. Pac Sci 58(4): 507-565.
Abstract:Tropical transpacific fishes occur on both sides of the world's largest
deep-water barrier to the migration of marine shore organisms, the 4,000- to
7,000-km-wide Eastern Pacific Barrier (EPB). They include 64 epipelagic oceanic
species and 126 species of shore fishes known from both the tropical eastern
Pacific (TEP) and the central and West Pacific. The broad distributions of 19
of 39 circumglobal transpacific species of shore fishes offer no clues to the origin
of their TEP populations; TEP populations of another 19 with disjunct Pacific
distributions may represent isthmian relicts that originated from New World
populations separated by the closure of the Central American isthmus. Eighty
species of transpacific shore fishes likely migrated eastward to the TEP, and 22
species of shore fishes (12 of them isthmian relicts) and one oceanic species
likely migrated westward from the TEP. Transpacific species constitute ~12%
of the TEP's tropical shore fishes and 15-20% of shore fishes at islands on the
western edge of the EPB. Eastward migrants constitute ~7% of the TEP's
shore-fish fauna, and a similar proportion of TEP endemics may be derived
from recent eastward immigration. Representation of transpacific species in
different elements of the TEP fauna relates strongly to adult pelagic dispersal
ability-they constitute almost all the epipelagic oceanic species, ~25% of
the inshore pelagic species, but only 10% of the demersal shore fishes. Taxa
that have multiple pelagic life-history stages are best represented among the
transpacific species. Among demersal teleosts that have pelagic larvae, pelagic
spawners are better represented than demersal spawners among transpacific
species, perhaps because offshore larval development and longer pelagic larval
durations provide the former with greater dispersal capabilities. There are
strong phylogenetic effects on representation in the transpacific fauna: (1) elasmobranchs
are proportionally better represented than teleosts, even teleosts
with more pelagic life-history stages; (2) a pelagic juvenile stage with great dispersal
potential allows tetraodontiforms that produce demersal or pelagic eggs
to be well represented; and (3) various speciose central Pacific families with
"adequate" larval dispersal characteristics lack transpacific species. El Niiios
potentially enhance eastward migration by increasing eastward flow and halving
transit times across the EPB. However, that effect may be offset by low productivity
and high temperatures in those eastbound flows. There is little clear
evidence of strongly increased migration across the EPB during El Niiios, including
recent extreme events (1982-1983 and 1997-1998). During such events
shore fishes in the TEP experience range expansions and become locally abundant at marginal areas such as the Galapagos, changes that can be confused with
increased migration across the EPB. Although there is a strong bias toward
eastward migration among the transpacific shore fishes, there likely is much
more westward migration than previously realized: 20-25% of transpacific species
may have migrated in that direction. Stronger eastbound than westbound
currents can account for this bias. Westward migrants have better developed
pelagic dispersal characteristics than many eastward migrants, suggesting that
westward migration is more difficult. Many westward migrants associate with
flotsam and flotsam-mediated migration is more likely to be westward. All
westward migrants occur at Hawai'i, but only about one-fifth of them at the
Marquesas. This bias may be due to: Hawai'i being a larger target and in the
path of most of the flotsam dispersal from the TEP; an eastward current that
impinges on the Marquesas, reducing westward arrivals; and most propagules
dispersing toward the tropical Marquesas originating in the temperate eastern
Pacific. However, the Hawaiian Islands also are much better sampled than the
Marquesas. Although the TEP reef-fish fauna may be depauperate relative to
that of the Indo-Malayan "center of diversity," it is as rich as the faunas of islands
on the western side of the EPB. Hence a preponderance of eastward migration
does not represent a response to a richness gradient across that barrier.
There is little evidence that a paucity of ecological groups in the native TEP
fauna is primarily responsible for the structure of the eastward-migrant fauna.
Rather, eastward migrants may simply represent a cross section of those in the
donor fauna, tempered by phylogenetic variation in dispersal ability. Because
few central Pacific fishes can live only on live corals and coral reefs, the rarity of
such reefs in the TEP is unlikely to strongly limit eastward migration. Differences
between oceanic and adjacent continental reef-fish faunas in the West
Pacific indicate that each is strongly tied to its respective habitat. Hence, the
rarity in the TEP of the (overwhelmingly) most abundant habitat present in the
central Pacific-tropical oceanic reefs-may strongly limit migration in both
directions across the EPB: there is little suitable habitat for eastward migrants in
the TEP and few suitable species and tiny source populations for westward migrants.
The global effects that oceanic/continental habitat differences have on
reef-fish biogeography need further assessment. Genetic data on ~18% of the
transpacific species indicate: that conspecific populations of oceanic species
(especially) and shore fishes are genetically well connected across the EPB; that
circumtropical taxa in the TEP include isolated isthmian relicts and recent
eastward migrants; that all five TEP species of one circumtropical genus (Thalassoma)
were derived by several eastward invasions after the closure of the
Isthmus of Panama; that some isolated Hawaiian central Pacific populations
were established by postisthmian invasion from the TEP; and that Indo-central
Pacific species unsuspectedly can co-occur with their endemic sibling sisters
in the TEP. Genetic data support distributional data that indicate a strong
preponderance of eastward migration across the EPB but also more westward
migration than previously thought. Future genetic studies should resolve
a question that distributional data cannot: how many widespread presumed
eastward-migrant transpacific species actually originated by westward migration
from the TEP?
Appears in Collections: Pacific Science Volume 58, Number 4, 2004

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